The use of low-voltage (e.g. less than equal 48V) control systems to operate domestic heating, ventilation and air conditioning (HVAC) systems is common. FIG. 1 is a schematic representation of a typical prior art two-wire control system 100. In their simplest form these control systems historically have consisted of a low-voltage alternating cycle (AC) source 105 (e.g. a transformer) located proximate to a HVAC component 110 (e.g. a furnace), a control unit 115 (e.g. a thermostat) located in the living area of a home and a two-wire conductor 120 interconnecting the transformer 105, the thermostat 115 and the furnace 110. The conductor 120 is arranged to supply AC current from the transformer 105 to the thermostat 115 via a first wire 122. The thermostat 115 switches the AC current typically using mechanical means 125 (e.g. a mercury bulb switch). The switched output of the thermostat 115 is connected via a second wire 124 to the furnace 110 to provide control (i.e. to signal a demand for heat). A current return path is provided between the furnace 110 and the transformer 105.
With the advancement of electronics technology and a desire for greater energy efficiency, the analog thermostats having mechanical switching means are being replaced by digital thermostats some of which include solid-state switching means. Where it is desired to replace an analog thermostat with a digital thermostat in a home having a two-wire conductor as described above, an issue exists with regard to providing a ground reference for the digital thermostat. Normally when no heat is being demanded the output of the thermostat would be in an open circuit state and therefore no current return path to the transformer would exist to provide a ground reference for the thermostat.
Manufactures of digital thermostats have typically addressed the lack of ground reference using one of two approaches. The first approach is to provide the digital thermostat with an independent power source (e.g. disposable dry-cell batteries) that powers the control logic and only switching the AC current for control signal purposes. This solution is not desirable in some situations (e.g. when the home is unattended for long periods of time) as a failure of the independent power source (e.g. when the batteries are discharged) causes failure of the thermostat. The second approach is to require that the two-wire be replaced (or alternatively supplemented) with a conductor having at least three wires. In some situations replacing the conductor is not practical or is too costly. In particular, replacement of the conductor is not a viable alternative when a replacement thermostat is being marketed to a homeowner who wants to do the installation himself.
Another related issue arises when additional HVAC equipment (e.g. heat-pump, air conditioning, humidifier) as added to existing HVAC equipment and the thermostat is replaced with a new thermostat having additional control capability for the added equipment. Typically at least one independent wire is required for each piece of equipment in addition to one wire for supplying AC current. In existing homes the existing conductor, even when it contain more than two wires, may not have sufficient independent wires for all of the equipment. One solution is to replace (or supplement) the existing conductor but, as discussed above, in some situations replacing the conductor is not practical or is too costly.
What is needed is a mechanism to allow the use of a low-voltage control unit (e.g. thermostat) requiring at least a given number (‘N’) of independent connections (i.e. wires), to control one or more pieces of equipment, with an interconnecting conductor having less than ‘N’ wires.